Good question,I bought a 21sf and then wanted more bang for the buck,bought the 20 and was very impressed and it's just so different than a 45acp round. The 10mm is to much fun(more expensive) but worth it. If i had to keep one it would be the 10mm....nasty! Love it! So much I bought a 29......even more fun in a smaller package.....it's a disease.......good luck.